Turbocharging refers to methods of increasing the air or air/fuel mixture density of a motor vehicle engine by forced induction of the intake air stream into the engine cylinders. A turbocharger uses an air intake compressor that is powered by the engine exhaust stream to increase the pressure of the intake air. Turbocharging is particularly desirable because it provides a fuel-efficient method by which the power output of the engine may be increased on demand. Similarly, for a given power output requirement, a turbocharged engine may have a smaller displacement than an engine which does not utilize forced induction.
A turbocharger generally consists of a turbine wheel and a compressor wheel mounted on opposite ends of a shaft. These components are each isolated within a turbine housing that is configured to direct the exhaust stream from the engine across the blades of the turbine wheel causing it to spin thereby driving the shaft and the compressor wheel disposed thereon. Rotation of the compressor inducts air into the compressor housing where it is pressurized and then output to the engine intake manifold.
Because the speed of the compressor is dependent on the pressure of the exhaust gas stream driving the turbine wheel, there is frequently not enough pressure at the beginning moments of vehicle acceleration to provide the desired pressurization of the intake air causing turbo “lag”. Furthermore, there is also frequently too much pressure at high engine output and peak engine RPM's that can cause over pressurization sufficient to cause damage to the engine and the turbocharger, such that a waste gate is commonly used to vent exhaust gas to avoid over pressurization of the system. These considerations have led to various approaches to provide more optimized output of the turbocharger, particularly in engines having separate cylinder banks, such as various opposed and V-type cylinder configurations.
One such approach has been the use of adjustable nozzles or vanes to provide increased pressure at low engine RPM's and exhaust output, which otherwise cannot be realized with a fixed geometry turbocharger. However, because of the relative complexity and cost of turbochargers incorporating variable nozzles, use in gasoline engines and small diesel engines has been limited. Another approach has been to incorporate twin turbochargers, one turbocharger for each cylinder bank, frequently oriented with turbine shafts rotating in opposite directions. Twin turbocharger configurations have been used in both outboard exhaust and inboard exhaust configurations. While the performance of twin turbocharger systems has been generally acceptable, they are generally undesirable because of the additional under-hood space required to house the additional turbocharger, particularly in the case of inboard twin turbochargers where both turbochargers must be housed in the space between cylinder banks, and also because of the cost of the additional turbocharger. Still another approach has been the use of various single turbocharger configurations, including both outboard exhaust and inboard exhaust configuration. Those in outboard configurations are cost effective but less efficient due to the necessity of routing the exhaust from the cylinder bank opposite the bank with which the turbocharger is associated. Furthermore, such designs are difficult to package under-hood due to the necessity of routing exhaust as described above. Likewise, single inboard turbochargers have been employed, but exhaust flow to the turbocharger is often compromised due to difficulty in proper routing of the exhaust gas from each of the cylinder banks to the turbine inlet, particularly where the exhaust ports are located outboard of the cylinder banks. With regard to the use of single turbochargers, various twin scroll designs have been proposed, however, such designs also have had the limitations described above associated with packaging of the turbocharger and routing of the exhaust to the turbine inlet. Further, such designs, while utilizing two scrolls, have placed them in symmetric configurations such that the inlet openings for both scrolls are positioned at the same radial location with reference to the center of rotation of the turbine wheel. This has necessitated that the exhaust gas stream or streams be channeled to this singular location for passage into the scrolls in conjunction with the operation of the turbine wheel, thus further constraining the routing of the exhaust conduits, which typically necessitates incorporation of sharp bends and other restrictions which affect the flow of the exhaust gases into the scrolls, thereby reducing the efficiency of the turbocharger and the engine.
In view of these and other limitations, improved turbine housing and turbocharger designs are desirable, as well as engines that are adapted to incorporate such designs.